1. Field of the Invention
This invention relates to a continuous distillation apparatus and, more particularly, to an inert gas-stripping and distillation apparatus and method whereby a recycled carrier gas is used to assist in evaporating a low-boiling-point component from a multicomponent system.
2. The Prior Art
Distillation and, more particularly continuous distillation, is the single most important separation unit operation in the process industry. It is used widely to upgrade feedstocks, separate reaction intermediates, and purify products in processes ranging from cryogenic separation of oxygen, nitrogen, and helium to the recovery of aromatics from coal. The term "distillation" is occasionally employed for those processes where a single constituent is vaporized from a solution, for example, in "distilling" water. In general, however, this term is properly applied only to those operations where vaporization of a liquid mixture yields a vapor phase, and it is desired to produce two or more vapor and/or liquid products. Thus, the separation of alcohol and water into its components is distillation; whereas the separation of brine into salt and water is evaporation, even in those cases where the salt is not desired and condensed water vapor is the only valuable product.
An evaporator is primarily a piece of equipment in which a liquid is boiled. The conditions under which evaporation is carried out in practice are widely varied. The liquid to be evaporated may be less viscous than water, or it may be so viscous that it can hardly flow. It may deposit scale on the heating surface; it may precipitate crystals; it may tend to foam; it may have a very high boiling-point elevation; or it may be damaged by the application of too high temperatures.
On the other hand, a rigid definition of drying that would sharply differentiate drying from evaporation is difficult to formulate. The term "drying" usually infers the removal of relatively small amounts of water from solid or nearly solid material while the term "evaporation" is usually limited to the removal of relatively large amounts of water from solutions. In drying processes, the major emphasis is usually on the solid product. In most cases, drying involves a removal of water at temperatures below its boiling point, whereas evaporation means the removal of water by boiling a solution. Another distinction is that in evaporation the water is removed from the material as practically pure water vapor, mixed with other gasses only because of unavoidable leaks. In drying, water is usually removed by circulating air or some other gas over the material in order to carry away the water vapor; but in some drying processes no carrier gas is used. It is understood for mixtures where water is replaced by some other liquid that the previous definitions also usually hold. While the foregoing definitions hold in many cases, there are also notable exceptions to every one of them. In the last analysis, the question of whether a given operation is called distillation, evaporation, or drying is largely a question of common usage.
In a recent study, distillation was determined to be an important energy consumer in almost every refinery and chemical plant. In petroleum refineries, which are the largest energy consumers on a per-plant basis, crude and vacuum distillation alone accounts for between 22.5 percent and 51 percent of the total energy consumed by the plant. In the conventional distillation column, energy in the form of heat is applied in a reboiler and removed through a condenser, with the operating temperature of the reboiler being substantially higher than the operating temperature of the condenser. Thus, the thermal energy is introduced into the distillation column at the highest temperature end (the reboiler) and removed from the distillation column at the lowest temperature end (the condenser). The resulting temperature differential between the reboiler and the condenser means that the separation of components is always accompanied with a degradation of energy even when heat leaks and other losses are excluded. Accordingly, the conventional distillation process, when viewed as a thermodynamic process, is notoriously inefficient with values as low as 1.9 percent having been reported for the thermodynamic efficiency of industrial distillation columns.
A conventional, continuous distillation or fractionating apparatus includes a column having a lower, stripping section and an upper, rectifying section. A feed of known concentration is fed at or near its boiling point to a location between the two sections. The distribution plate at which the feed enters is referred to as the feed plate with all plates above the feed plate constituting the rectifying section and all plates below the feed plate constituting the stripping section. The liquid feed flows downwardly through the stripping section to the bottom of the column where a definite liquid level is maintained. Liquid is removed from the stripping section and directed to a reboiler to produce vapor, which returns to the lower end of the stripping section. The bottom product or residue may undergo further processing or may be directed through a cooler to serve as a preheater for the feed stream. The vapor produced in the reboiler passes upwardly through the stripping section and rectifying section and is removed from the column where it is partially or totally condensed in a condenser. Condensate is collected in an accumulator where a portion of the condensate is returned as reflux liquid to the top plate of the rectifying section. The reflux provides the down-flowing liquid in the rectifying section to achieve the necessary rectification since no rectification would occur in the rectifying section without the reflux and the concentration of the overhead product would be no greater than the vapor rising from the feed plate. Condensate from the accumulator that is not returned as reflux is withdrawn as overhead product and may be cooled in a product cooler. If no azeotropes are encountered, both overhead and bottom products may be obtained in any desired degree of purity if enough plates and adequate reflux are provided.
The foregoing, conventional distillation apparatus requires relatively high temperatures to produce the desired separation. However, it would be an advancement in the art to provide a distillation apparatus and method that utilizes thermal energy from a low-temperature energy source for the removal of a lower-boiling-point constituent from a mixture. It would also be an advancement in the art to provide a combination distillation/evaporating/drying apparatus and method for removing an alcohol from a fermented residue. It would also be an advancement in the art to provide an apparatus and method for separating a low-boiling-point liquid by advantageously combining the various features of distillation, evaporation, and drying unit operations. Such a novel invention is disclosed and claimed herein.